1. Field of the Invention
The present invention relates to a control unit for an electric power steering apparatus that provides steering assist force by motor to the steering system of an automobile or a vehicle. The invention particularly relates to a control unit for an electric power steering apparatus that generates a current dither signal for removing the influence of static friction in a completely stationary status of the motor.
2. Description of the Related Art
An electric power steering apparatus for applying assist load to the steering apparatus of an automobile or a vehicle using turning effort of a motor applies the driving force of the motor to a steering shaft or a rack axis based on a transmission mechanism like gears or belts via a reduction gear. Such a conventional electric power steering apparatus carries out a feedback control of a motor current for accurately generating an assist torque(a steering assist torque). The feedback control is for adjusting a motor application voltage so as to minimize a difference between a current control value and a motor current detection value. The motor application voltage is generally adjusted based on a duty ratio of a PWM (Pulse Width Modulation) control.
A general structure of an electric power steering apparatus will be explained with reference to FIG. 1. A shaft 2 of a steering wheel 1 is connected to a tie rod 6 of running wheels through a reduction gear 3, universal joints 4a and 4b, and a pinion rack mechanism 5. The shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1. A motor 20 for assisting the steering force of the steering wheel 1 is connected to the shaft 2 through a clutch 21 and the reduction gear 3. A control unit 30 for controlling the power steering apparatus is supplied with power from a battery 14 through an ignition key 11. The control unit 30 calculates a steering assist command value I of an assist command based on a steering torque T detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12. The control unit 30 then controls a current to be supplied to the motor 20 based on the calculated steering assist command value I. The clutch 21 is ON/OFF-controlled by the control unit 30, and is kept ON (connected) in an ordinary operation status. When the control unit 30 has decided that the power steering apparatus is in failure, and also when the power source(voltage Vb) of the battery 14 has been turned OFF with the ignition key 11, the clutch 21 is turned OFF (disconnected).
The control unit 30 is mainly composed of a CPU. FIG. 2 shows general functions to be executed based on a program inside the CPU. For example, a phase compensator 31 does not show a phase compensator as independent hardware, but shows a phase compensation function to be executed by the CPU.
Functions and operation of the control unit 30 will be explained below. A steering torque T detected by the torque sensor 10 and then input is phase-compensated by the phase compensator 31 for increasing the stability of the steering system. The phase-compensated steering torque TA is inputted to a steering assist command value calculator 32. A vehicle speed V detected by the vehicle speed sensor 12 is also inputted to the steering assist command value calculator 32. The steering assist command value calculator 32 calculates a steering assist command value I as a control target value of a current to be supplied to the motor 20, based on the inputted steering torque TA and the inputted vehicle speed V. The steering assist command value I is inputted to a subtractor 30A, and is also inputted to a differential compensator 34 of a feedforward system for increasing a response speed. A difference (Ixe2x88x92i) calculated by the subtractor 30A is inputted to a proportional calculator 35, and is also inputted to an integration calculator 36 for improving the characteristic of a feedback system. Outputs from the differential compensator 34 and the integration calculator 36 are inputted to an adder 30B, and are added together by the adder 30B. A result of the addition by the adder 30B is obtained as a current control value E, and this is inputted to a motor driving circuit 37 as a motor driving signal. A motor current value i of the motor 20 is detected by a motor current detecting circuit 38, and the motor current value i is inputted to the subtractor 30A and is fed back.
According to a widely-distributed hydraulic power steering apparatus, the apparatus has a characteristic that the friction of a cylinder section increases in proportion to a cylinder pressure P (a horizontal axis T represents a steering torque), as shown in FIG. 3. The apparatus has hysteresis because of the frictional characteristic. When a vehicle is cornering, for example, the hysteresis prevents the steering wheel from being suddenly returned by a self-aligning torque. This improves the steering of the driver. FIG. 4 shows this status. When the steering torque T has suddenly changed by A T, the cylinder pressure P changes by P1 in the absence of hysteresis. However, in the presence of hysteresis, the cylinder pressure P changes by P2( less than P1). Therefore, in the presence of hysteresis, it is possible to make smooth the change in the cylinder pressure P in relation to a change in the steering torque T. It has been known that the hysteresis width changes according to a size of friction. In the case of a rubber packing of a hydraulic cylinder, the rubber is compressed along an increase in the cylinder pressure. The hysteresis width increases based on an increase in Coulomb friction. It is important for the steering that the driver feels strong self-aligning torque at a neutral point, and does not feel so strong self-aligning torque when the vehicle is cornering. In this sense, it is ideal that, like in the hydraulic power steering apparatus, the friction(hysteresis) becomes small in an area of a small steering angle xcex8, and the friction (hysteresis) becomes large in an area of a large steering angle xcex8.
On the other hand, according to an electric power steering apparatus, the apparatus has constant friction independent of the assist torque T, as shown in FIG. 5. FIG. 6 shows a relationship between a motor angular velocity xcfx89 and friction F. Static friction works in an area xe2x80x9cAxe2x80x9d(xe2x88x92xcfx891 to +xcfx891) where the motor angular velocity xcfx89 is small. In other angular velocity areas (lower than xe2x88x92xcfx891, and higher than +xcfx891), the friction F appears as an addition of Coulomb friction B of a constant value (+F2, xe2x88x92F2) and viscous friction C that increases two dimensionally along an increase in the motor angular velocity xcfx89.
From the relationship between the motor angular velocity xcfx89 and the friction F shown in FIG. 6, the friction characteristic becomes discontinuous at a position where the motor angular velocity xcfx89 is near to zero. Therefore, there is an inconvenience that when the steering angle has become completely stationary at certain timing, the driver feels that the steering wheel has been fixed to this position. The electric power steering apparatus carries out various kinds of compensation, including the compensation for the motor inertia for improving the steering performance, and compensation for removing the influence of friction. As the above compensation control is carried out based on the motor angular velocity, it is not possible to remove the influence of a status that the motor is completely stationary. In other words, it is not possible to remove the influence of the static friction of the electric power steering apparatus.
The present invention has been made in the light of the above situations. It is an object of the present invention to provide a control unit for an electric power steering apparatus capable of obtaining continuous and stable feeling of steering, by removing the influence of static friction in a completely stationary status of the motor of the electric power steering apparatus.
In order to achieve the above object, according to one aspect of the present invention, there is provided a control unit for an electric power steering apparatus that controls a motor for giving steering assist force to a steering mechanism, based on a current control value calculated from a steering assist command value calculated based on the steering torque generated in the steering shaft, and a current value of the motor. Particularly, the control unit has a current dither signal generating unit for generating a current dither signal when the motor angular velocity is within a predetermined range of angular speed, and for adding the current dither signal to the steering assist command value.
According to another aspect of the invention, there is provided a control unit for an electric power steering apparatus of the above aspect, wherein the predetermined value is an angular velocity of the motor corresponding to the static friction or the current dither signal is expressed as Kxc2x7sin xcfx89ot, where K is a constant and xcfx89o represents a dither angular frequency.